The present invention relates to an optical magnetic recording and reproducing system capable of recording information on a magneto-optical disk at high density.
In order to record information in the optical magnetic memory system, a laser beam is focused on the magneto-optical disk to elevate the temperature of a point to be recorded. On the other hand, the disk is magnetized, thereby rendering the direction of the magnetic moment in a magnetic domain of the disk upward or downward. Thus, the information is represented by one or zero digital signals.
FIG. 5 shows an example of a conventional optical magnetic recording and reproducing system employing the magnetic field modulation system. In order to record information on a magneto-optical disk 1, a laser beam from a laser device L is focused on a recording surface of the disk 1 through an objective lens 2. The beam reflected from the disk 1 is supplied to a focus error detector 4 through a beam splitter 3. The focus error detector 4 detects the focus error in accordance with the reflected beam, and applies a focus error signal to a focus servo 5. The focus servo 5 in turn applies a focus servo signal corresponding to the focus error signal to a driver 6. The driver 6 applies an exciting current in response to the servo signal to an actuator coil 7. The actuator coil 7 accordingly generates a magnetic field. The actuator coil 7 and hence the objective lens 2 are moved along the optical axis of the objective lens 2 due to a magnetic force of an actuator magnet 8.
When the laser beam is thus correctly focused on the recording surface of the disk 1, the temperature of the recording point thereon rises. At the same time, a signal current is applied to an excitation coil 11 which is wound around a core 10 mounted on a fixed magnetic head 9, rendering the direction of the magnetic moment in a magnetic domain of a ferro magnetic material formed on the recording surface of the disk upward or downward. Thus, information represented by one or zero digital signals is stored.
When reproducing the recorded information, a lighter laser beam correctly focused through the focus servo system is emitted. The beam is reflected on the disk 1, and the direction of the polarization plane of the reflected beam is different in dependency on the direction of the magnetic domain, so that whether the reflected beam is one or zero can be determined. Hence, the information is reproduced as shown by a waveform shown in FIG. 6.
FIG. 7 shows another conventional optical magnetic recording and reproducing system provided with a movable magnetic head 9. The magnetic head 9 is provided with an actuator coil 13 which generates a magnetic field in accordance with a servo signal from a magnetic head driver 12, and an actuator magnet 14.
The recording and reproducing system is constructed to move the magnetic head 9 to maintain the distance between the disk 1 and the magnetic head 9 constant in spite of the vibration or warp of the disk 1. Thus, the intensity of the magnetic field caused by the magnetic head 9 is kept constant, thereby reproducing information as shown by a waveform in FIG. 8.
However, in the above described magnetic field modulation method, a resultant magnetic field is generated due to a leakage of magnetic force from the actuator coils 7 and 13 and actuator magnets 8 and 14. The resultant magnetic field affects the recording of the information, so that the magnetic field for recording the information is distorted.
Comparing the FIGS. 6 and 8, the magnitude of the distortion in the system having the fixed magnetic head is larger than that having the movable magnetic head. This is caused by the fact that in the system with the fixed head, the distance between the disk 1 and the magnetic head 9 fluctuates when the disk 1 vibrates, so that the magnetic field for recording the information also fluctuates. Consequently, when the magnetic field for recording the information is distorted, the information can not be accurately recorded or reproduced, thereby decreasing the reliability of the system.